Modulation of the resonant frequency of a circuit using an energy field

ABSTRACT

A transponder has a resonant RLC circuit with one or more electromagnetic energy storage components that vary in response to an externally applied modulating energy field. In addition to the externally modulating energy field, a base station transmits a carrier signal with a frequency essentially the same as the quiescent resonant frequency of the RLC circuit. As the component(s) vary, the resonant frequency of the RLC circuit changes, modulating the carrier signal with the external modulating energy field. Effects of the modulation are detected by the base station. Information (e.g., the presence of a tag) is obtained by receiving and demodulating the modulated signal at the base station. One or more of the circuit elements (e.g. different preferred embodiments of one or more capacitors, inductors, and resistors) can be varied (e.g. mechanically) to modulate the carrier signal. This allows the resonant circuit to modulate the carrier signal with multiple modulation frequencies to encode multiple bits of information on the carrier.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of patent application Ser. No.08/569,375, filed Dec. 8, 1995, now U.S. Pat. No. 5,812,065, which is acontinuation-in-part of patent application No. 08/514,705 filed on Aug.14, 1995, now abandoned.

FIELD OF THE INVENTION

This invention relates to the field of modulating a carrier frequencywith a modulating signal. More specifically, the invention relates tothe modulation of a carrier frequency of a resonant circuit used inradio frequency tagging.

BACKGROUND OF THE INVENTION

A resonant circuit is one in which the values of circuit resistance, R,capacitance, C, and inductance, L, are chosen such that the reactance ofthe resonant circuit is a minimum at a resonant frequency.

In amplitude modulated radio reception, the antenna circuit is tuned toresonate with the carrier frequency of the particular radio frequencycircuit. This tuning to resonance or near resonance of the RF antennacircuit provides a means for discriminating against the many othercarrier frequencies used by other broadcasting stations, therebyallowing the listener to choose one station without interference fromthe others broadcasting simultaneously.

Information content is impressed on the carrier via a mixing schemeknown as modulation in which a nonlinear clement combines the carrierwith the frequencies containing the information to yield a sum anddifference frequency signal. This mixing process takes place at thebroadcast station. The information is subsequently retrieved by anamplitude modulated radio receiver which separates the carrier viarectification and filtering leaving only the information signal, amethod generally referred to as demodulation. Other types of modulatingand demodulating circuits are well known, e.g., circuits using frequencymodulation, and pulse modulation.

In prior Radio Frequency (RF) tagging art, a resonant circuit isdisposed on a thin insulating dielectric substrate to form a tag for usein electronic article detection (EAS) schemes. Generally, the coil ofthe resonant circuit consists of a closed loop of a conducting elementwhich has a certain value of resistance and inductance. A capacitiveelement which forms part of this closed loop consists of two separateareas of thin metal conducting film disposed on opposite sides of thedielectric. The tag is attached to articles to be protected from theft.An RF signal at or near the resonant frequency of the resonant circuitis emitted from a base station. When the tag is in the RF field, thetag's absorption can lead to a change in the tank circuit current of thebase station and a power dip in a receiving coil. Either one of thesetwo effects can be used to sense the presence of the tag and hence theitem to which it is attached. Thus, an alarm can be made to sound wheneither of these effects are sensed by a pickup coil or by an amplifier,indicating improper removal of an item. To deactivate the tag, arelatively high RF power pulse can be applied at the counter at whichthe point-of-sale of the item takes place. This high power acts to shortthe capacitor or burn out a weak portion of the coil. In either case,the circuit is no longer resonant and will not respond to the RFinterrogation from the base station. In that case, the customer who hasmade a legitimate purchase at the point-of-sale counter can pass throughthe interrogation-sensing gate without setting off an alarm. It is clearfrom this description that these tags, once deactivated, arc notreusable. In addition, in the configuration just described, the tags arecapable of only conveying one bit of information. Thus, they cannot giveany information regarding the item's identification and are useful onlyfor anti-theft applications. This kind of tag is normally classified asa single bit tag.

Some RF tags consist of a resonant coil or a double sided coilcontaining two thin film capacitors with the plate of each capacitor onopposite sides of the dielectric. Such tags can be used for sourcetagging and have an initial frequency that is different from thefrequency used at the retail establishment for theft protection. Forexample, in U.S. Pat. No. 5,081,445 (assigned to Checkpoint), the tag isdesignated as being in a deactivated state until the first capacitor isshorted by means of a high power RF pulse at the then resonantfrequency. Disabling the capacitor shifts the resonant frequency of theRF circuit to the store interrogation frequency. A second deactivationpulse is used to disable the second capacitor at the point-of-sale whenpayment is received for the item to which the tag is attached. At thisstage, the tag is no longer usable and has been permanently destroyed.

Some additional art discloses two or more frequencies that can beobtained on a RF coil tag by altering the capacitance of the circuit. Inone case, a strong DC electric field is applied to change the effectivedielectric constant of the capacitor. Thus, the circuit has two resonantfrequencies depending on the value of the applied electric field. Due tothe ferroelectric hysteresis, the tag can be deactivated by theapplication of a DC field. However, it can also be reactivated and hencere-used by applying a DC field of opposite polarity (U.S. Pat. No.5,257,009, assigned to Sensormatic). In an earlier embodiment, a set ofcapacitors connected in parallel attached to an inductance have beendescribed in U.S. Pat. No. 5,111,186 assigned to Sensormatic in whicheach dielectric of the set of capacitors varies in thickness. In thismanner, a series of resonant frequencies can be obtained by applyingdifferent voltages (electric fields). Each of the capacitors thenchanges capacitance at a different electric field (voltage) levelsdepending on the thickness of the dielectric. There can be some concernregarding the high voltages required for creating the change in thedielectric. Also, using this apparatus, the remanent state offerroelectric tends not to be very stable for long periods of time.Additional concern relates to the dielectrics, which are alsopiezoelectric materials which have properties quite sensitive to stress.

U.S. Pat. No. 5,218,189, assigned to Checkpoint, provides an array ofseries capacitors connected in parallel with an inductor. Here, theresonance can be altered by selectively shorting one or more of thecapacitors, thereby changing the resonant frequency of the resultingcircuit. A frequency code can thereby be established by disabling orburning out selective capacitors at the time of interrogation, thosecapacitors becoming disabled which at the time of manufacture of the tagwere “dimpled”. The disadvantage is that the item or person is subjectto high r.f. fields during interrogation. Also, the range of frequenciesthat needs to be scanned is necessarily large. This makes detectiondifficult since the requirement to scan a large band of frequencies putsa strong demand on the flat response of the detector circuit.

U.S. Pat. No. 4,745,401 describes an embodiment for a reusable tag. Itis comprised of two ferromagnetic elements, one soft (low coercivity)and one hard (high coercivity) both physically covering a portion of anR.F. coil. The ferromagnetic element with high coercivity can bemagnetized to apply a bias field to the soft material to put the latterinto saturation. In that state, the R.F. field generates very smallhysteresis losses leading to a relatively high Q of the tag circuit. Onthe other hand, when the hard magnet is demagnetized, the RF Fieldresults in hysteresis losses in the soft material which lowers the Q ofthe circuit. This change in Q can be used to determine whether a tag isactive or has been deactivated. While this constitutes a reusable tag,the change in Q tends to be small and thereby some what more difficultto distinguish from other effects that attenuate the absorption.

In U.S. Pat. No. 3,500,373 an apparatus is described for interrogatingand sensing the presence of a RF resonant tag. Here the interrogatingfrequency is swept around a center frequency. In general, there is verylittle radiation emitted except when the tag is present in the field oftile emitter. Thus, when there is no tag in the antenna field, verylittle energy is lost from the antenna circuit. When the swept frequencycoincides with the resonant frequency of an active tag, energy isabsorbed and a sensing circuit detects a drop in voltage level in theinterrogating antenna oscillator circuit. The tag absorption occurstwice with every complete sweep cycle resulting in a negative dip in theoscillator circuit. The negative dip causes pulse modulation which isfiltered, demodulated and amplified to cause an alarm to be activated,indicating theft of an item. Thus, the basic detection is achieved byvarying the interrogation carrier frequency to match the resonance of atag whose center frequencies span a range depending on the type or makeof tag.

STATEMENT OF PROBLEMS WITH THE PRIOR ART

As already stated, the above prior art senses changes in the resonantstate of a tag, either by changing its Q or changing the frequency ofthe tag circuit. The cited embodiments are detected by way of sensingthe change in the magnitude of the tag absorption at the resonantfrequency or a change in the Q of the tag circuit. However, since thesetags generally operate in the MHz regime, they are easily shielded sothat the signal both to and from the tag is readily attenuated. Sincesensing the tag relies strongly on the absorption or the carrier and thefrequency spectrum of the pulse occurring during the frequency sweep,pulse detection has difficulties under a variety of conditions. Sweepingintroduces harmonics or higher frequencies so that the signal to noiseis degraded. Therefore, the demodulation of the carrier containing thepulse as a result of the tag in the field of the swept carriernecessarily contains more noise than a carrier modulated by a quasi cwsine wave. For some tags, it may also be hard to distinguish a change inQ from a change in position or orientation of the tag relative to the RFfield direction. Therefore, these prior art tags can produce weaksignals that are difficult to discriminate at the base station(transceiver).

Many of the prior art RF tags are limited to one bit of information andare not reusable. Many of the reusable tags in the prior art require theuse of very strong fields.

OBJECTS OF THE INVENTION

An object of this invention is an improved radio frequency (RF) tagtransponder.

An object of this invention is an improved, reusable RF tag with one ormore bits of information.

Another object of this invention is an improved, reusable RF tag thatcreates a dependable and easy to discriminate signal at the basestation.

An object of this invention is a tag with a reusable resonant circuitthat is capable of modulating a carrier signal with one or more discreteand highly detectable modulating frequencies of an applied energy field.

SUMMARY OF THE INVENTION

The present invention is a transponder apparatus that uses a resonantcircuit with one or more variable circuit components. The resonantcircuit has a resonance frequency at or near the frequency of a radiofrequency (RF) carrier.

A system and a method can include a base station that communicates withthe transponder by using an RF carrier.

The base station interrogates the resonant circuit by using an RFcarrier signal with a frequency at which the resonant circuit resonatesand a modulating signal that cause one or more components of theresonant circuit to vary at a component frequency there by varying theresonance frequency of the resonance circuit. As the resonance frequencyof the resonance circuit varies, the RF carrier signal is modulated bythe modulating signal.

The values of one or more of the components (resistance, inductance, andcapacitance) or the resonant circuit are varied by one or more remotemodulating signals. The modulating signals are transmitted from alocation (or locations) remote from the resonant circuit. The modulatingsignals are external energy fields that vary continually withoutinterruption over a period of time in magnitude and/or frequency at amodulating signal frequency. For example, the modulating signal can bean acoustic field or an audio frequency electromagnetic field.

In many of the preferred embodiments, the varying component value(s) ofthe resonant circuit vary due to a mechanical change of the component.The mechanical change of the component (at the component frequency), andtherefore the change in value of the component, varies most when thefrequency of the force produced by the modulating signal(s) is equal toone of the mechanical resonances of the varying component(s) of theresonant circuit.

The modulated RF carrier is demodulated, typically at the base station,by using a receiver (demodulator) tuned to demodulate the carrier toobtain the encoded information (like the modulating signal) from thecarrier.

Various novel preferred embodiments of these variable components of theresonant circuit are disclosed, including capacitors with vibratingplates (physically constrained in various ways, e.g. as a cantilever,sliding plate, etc.); inductors with variable permeability and/or mutualinductance; and variable resistance.

In a preferred embodiment, the resonant circuit is used in a radiofrequency (RF) tag. Various preferred embodiments of the RF tag includemeans for establishing a code of one or more bits by introducing one ormore variable components of the RLC circuit. Each component hasmechanical resonance or a state that responds more at one of themodulating signal frequencies than at other frequencies. An RF tag withmultiple bits is made by using components of the RLC circuit withdifferent states on a single RF tag.

In a preferred embodiment, the system and tag transponders are used foranti-theft protection as well as item identification. In addition, thesystem includes means for interrogating and detecting the device toretrieve the information carried by the tag.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of preferredembodiments of the invention with reference to the drawings as follows:

FIG. 1 is a diagram of hardware for a preferred base station used toexcite a novel remote transponder having a resonant circuit.

FIG. 2A shows the RF resonant circuit where the variable componentelement is a variable capacitor made to vary by the modulatingsignal(s).

FIG. 2B is a side view of a cantilever used as one plate of a preferredembodiment of the variable capacitor component, where the cantilever iscaused to vibrate by the modulating signal(s).

FIG. 2C is a side view of a bar with both ends clamped acting as oneplate of a preferred embodiment of the variable capacitor component,where the clamped bar is caused to vibrate by the modulating signal(s).

FIG. 2D is a side view of a free (unconstrained) bar acting as one plateof a preferred embodiment of the variable capacitor, where the free baris caused to vibrate by the modulating signal(s).

FIG. 2E is a side view of a first and a second cantilever plate that areattracted to one another by a time varying force due to a time varyingmagnetic field.

FIG. 3 shows a preferred structure in which at least one vibrating plateof the variable capacitors described in FIG. 2 comprises a bimorph.

FIG. 4 is a multibit resonant circuit comprising several variablecapacitors components, each varying its capacitance a greatest amount ata distinct frequency of modulating signal(s) and in combination causingthe resonant circuit to resonate at a circuit resonance frequency.

FIG. 5A (side view) and FIG. 5B (top view) are drawings of a preferredembodiment of a variable capacitor component that can vary capacitanceby sliding at least one of the two capacitor plates.

FIG. 5C is a drawing of a preferred embodiment of a variable capacitorcomponent with a sliding plate that has a mechanically coupled restoringforce.

FIG. 5D is a drawing of a variable capacitor component with one or moresliding plates that change a small gap as they slide to varycapacitance.

FIG. 5E is drawing of a capacitor component with sliding elementsconfigured to have magnetostrictive movement in at least twonon-parallel directions thereby making the capacitor variation lesssensitive to orientation with respect to the field created by themodulating signal.

FIG. 6A is a drawing of a circuit containing a variable inductancecomponent to modulate the resonant frequency of the resonant circuit.

FIG. 6B is a drawing of an alternative variable inductance modulatingthe resonant frequency of the resonant circuit.

FIG. 6C is a drawing of a variable mutual inductance modulating theresonant frequency of the resonant circuit.

FIG. 6D is a drawing of an alternative variable mutual inductancemodulating the resonant frequency of the resonant circuit.

FIG. 7A is a drawing of a resonant circuit with a variable impedancecomponent.

FIG. 7B is a drawing of a circuit with a variable impedance componentthat is primarily varies in resistance.

FIG. 8 is a block diagram of an interrogation detection system used withthe transponders/tags with, variable resonant circuits and a frequencymodulation (FM) embodiment.

FIG. 9 is a flow chart of a preferred method with steps for sending aninterrogation signal and analyzing the detected signal from thetransponder/tag.

FIG. 10 is a drawing of an illustration of an embodiment used to“personalize” the resonant circuit.

FIG. 11 is a drawing of a locking/unlocking mechanism for vibratingmembers used to “personalize” the resonant circuit.

DETAILED DESCRIPTION OF THE INVENTION

The present system (190) shown in FIG. 1 includes a noveltransponder/tag (150) comprising a resonant circuit (125). The resonantcircuit (125) has the fixed components of resistance, R (112),inductance L (113), and capacitance C (114) and one or more variablecomponents of resistance R (115), inductance L (116), and/or capacitanceC (117) which may be modified by an external modulating signal (103A).In addition, the system (190) includes a base station (175), providedfor interrogating the resonant circuit (125), used as a transponderand/or an RF tag (150).

The base station (175) includes a transmitter (109) for transmitting theRF carrier signal (104A) and the modulating signal (103A) and a receiver(110) for receiving and demodulating the modulated carrier signal (155),i.e., the carrier signal (104A) modulated with the modulating signal(103A) by the transponder/tag 150.

The transmitter (109) comprises an RF carrier generator (101), RFcarrier amplifier (100), RF carrier antenna (104), a modulating signalgenerator (102), a modulating amplifier (122), and modulating signalantenna (103). The RF carrier generator (101), amplifier (100), andantenna (103) produce the RF carrier (104A). Equipment like this is wellknown in the radio frequency transmission arts. The modulating signalgenerator (102) and the antenna (103) produce an external modulatingenergy field (103A), for varying one or more of the variable circuitcomponents (115, 116, 117) of the resonant circuit/tag (150). Since theenergy field (103A) is produced in the base station (175), it isproduced externally or remotely from the transponder/tag (150). Theexternally applied modulating field/signal (103A) can include anyacoustic, magnetic and/or electromagnetic field (103A) necessary to varythe variable components (115, 116, and 117) as described below. Themodulating signal generator (102) and the antenna (103) would bedesigned to produce this specific modulating signal (103A). For example,if the modulating signal (103A) is an acoustic energy field, the signalgenerator (102) would be an audio frequency electromagnetic signalgenerator and the antenna (103) would be a loud speaker. If themodulating signal (103A) is an electromagnetic field, the signalgenerator (102) would be an RF signal generator and the antenna (103)would be an RF antenna.

By varying/exciting these variable resonant circuit (150) elements(115,116,117) with the modulating signal (103A), the RF carrierfrequency (104A) is modulated to produce the modulated carrier signal(155). The modulated carrier (155) signal is sensed by a receiver (110)of the base station (175) through the receiving antenna (105). Thereceiver demodulator (106) demodulates the modulated carrier signal(155) and amplifies the demodulated signal (156) to retrieve theinformation, e.g., the modulating signal (103A) from the tag (150).Typically, this information indicates that a transponder/tag (150) ispresent in the field (103A, 104A) range of the base station (175)because only the transponder/tag (150) would modulate the carrier (104A)with the modulating signal (103A). Alternatively, the information can bea distinct frequency or frequencies that are modulated on the carrier(104A) by the transponder (150) as determined by the response of thevariable components (115, 116, 117) to the modulating signal (103A)which is swept through a range of frequencies. Generally, the retrievedinformation (156) is the demodulated signal derived from the carrier(104A) that was modulated at one or more modulating signals (103A) bythe resonant circuit of the transponder/tag (150).

A typical demodulating scheme will utilize a superheterodyne receiverand amplitude modulation. Any demodulated signal is transmitted via aninterface to a computer (107) to interpret the code, i.e., retrievalinformation (distinct frequency or frequencies) (156) impressed byresonance circuit (150) on the carrier (104A). This code is the presenceor absence of one or more modulating signals (103A) modulated or notmodulated on the carrier frequency (104A) by the resonant circuit (150).

In the quiescent state, i.e. when the modulating signal (103A) is notvarying any of the variable components (115-117), of the resonantcircuit (125) the values of the fixed circuit elements, R (112), L (113)and C (114) in combination with a quiescent value of each variablecomponent (115-117) determine the tag's resonant frequency which is alsothe frequency of the RF carrier signal (104A). To interrogate the tag,the carrier signal (104A) at carrier frequency (104A)is applied to thetag (150). Simultaneously, the modulating field or signal (103A) isapplied to alter the resonance frequency of the resonance circuit bychanging one or more of the variable components (115-117). Typically,the variable components vary at a component frequency, i.e., thecomponents vary continually in a period of time. As described in somepreferred embodiments below, the component frequency may or may not bethe same frequency as one of the mechanical resonance frequencies of thecomponent.

Disclosed below are alternative preferred embodiments of variablecomponents (115-117) of the resonant circuit (125) that are varied usingthe modulating signal (103A).

A preferred transponder/tag embodiment with a variable capacitorcomponent (117) is shown schematically in FIG. 2A. Here the variableelement is a variable capacitor (117). In alternative preferredembodiments, the capacitor component (117) variation can be achieved bychanging the spacing between the capacitor (117) plates and/or theoverlap of the two plates. In these capacitor configurations, theoverlap is the projection of one plate onto the second plate. Thus, thisoverlap region defines the plates of the modulating capacitor. Thevariable capacitor (117) shown in FIG. 2A, comprises two plates, atleast one that can move continually with time with respect to the otherdue to the modulating field/signal (103A) giving rise to a continuouschange in the spacing or gap between the plates and/or the overlap ofthe plates with respect to one another. In general the movement betweenthe plates can be parallel to the planes defined by the plates or at anyangle with respect to the plane of one of the plates. The movable platecan be constrained by boundary conditions to affect its response to themodulating signal (103A). One or more of such capacitors can beutilized, each of which can have a distinct response to the externalexcitation, i.e., the modulating signal (103A). In general, the platesof the capacitor are made from any electrical conductive materialincluding metals, like copper, etc.; conductive polymers; bimorphs (seebelow); etc.

FIG. 2B shows a typical parallel plate capacitor where one plate (201)is unmovably fixed to a substrate (201A), a second plate (202) isclamped at one of its ends (boundary condition) to an insulating support(203) that is also affixed to the substrate (201A) and/or the firstplate (201). In this arrangement, the top, movable plate (202) comprisesa cantilever with a well defined set of mechanical resonances whichdepend on the boundary conditions (clamped at one end) and thecantilever length, thickness, material density and bulk modulus. Ingeneral, such a cantilever can be excited by modulating signal (103A),e.g., an acoustic wave or magnetic field at any of the cantilevermechanical resonance frequencies. Therefore, when excited by themodulating signal (103A) with a frequency approaching one of thesemechanical resonance frequencies, the response, i.e. movement of thefree end or vibration, of the cantilever plate (202) varies thequiescent spacing or gap (200G) between the plates (201, 202), typicallyon the order of 250 microns. The spacing variation (207) is a maximumwhen the frequency of the modulating signal (103A) is equal to one ofthe mechanical resonance frequencies of the cantilever. Accordingly,when the frequency of the modulating field/signal (103A) is equal ornear to a mechanical resonance frequency of the the variable capacitor(117), in FIG. 2B a cantilever, the change in capacitance of thecapacitor (117) and hence the change in the resonance frequency of theresonant circuit (125) is the greatest.

Acoustic waves apply oscillatory pressure variations on the cantileversetting it into resonant vibration when the excitation frequency of themodulating signal (103A) coincides with mechanical resonant frequenciesof the cantilever. In this embodiment, an acoustic loudspeaker (103) andaudio oscillator (102) correspond respectively to the modulating signalgenerator (102) and antenna (103) in FIG. 1. Such mechanical excitation(modulating signal (103A)) will produce a change in the capacitance atthe frequency of the mechanical resonance of the cantilever andconsequently result in a change of the tag's (150) unperturbed orquiescent state resonance. When the resonant circuit (125) is resonatingat the carrier frequency (104A) and the external modulating signal(103A) is applied at a frequency that causes modulating element (117) tomechanically resonate, the RF carrier signal (104A), along with the RFreflected signal (104R), the RF absorbed signal (104AB), and the RFtransmittal signal (104T), are modulated at the mechanical resonancefrequency, i.e., the frequency of the external modulating signal (103A).

See FIG. 2C. In place of the cantilever, a thin bar (204) of materialcan be used. This bar is mechanically clamped (203A) at both ends withan electrically insulating material (203A) such that its instantaneousspacing (207) due to its bending or bowing motion can be used to changethe capacitance (average spacing between the plates 200G) periodically.As before, the boundary conditions (clamping at both ends) determine themechanical resonance frequency of the capacitor (117).

See FIG. 2D. Another preferred embodiment is to have the bar free atboth ends (206) separated by a thin dielectric (205) from the oppositeplate (201) with an electrically contacting element (208) attached to anend of the free bar (206) in a manner that only minimally perturbs theboundary condition at that end. Here the capacitance change is broughtabout by the mechanical resonant vibrations of the bending modes whichresult in a change from the quiescent (200G) into the instantaneousspacing (207) between the bar and the fixed plate capacitive electrode.

The dielectric (205) can be made of any one of a number of polyimides,for example.

For certain RF tag applications, the capacitor (117) can be disabled inthe conventional manner by shorting out the main capacitor or part ofthe coil (113) using a high power RF signal, generally at thepoint-of-sale.

Note that in these embodiments, the capacitor plates can be any shape.Further, other constraints on the movable bar or alternatively a plateare possible. For example, when the element is a plate (206), as in FIG.2D, it can be constrained at one or more edges or at one or more pointsinternal to the plate, like the center. By changing these mechanicalconstraints, the mechanical resonance frequency of the time dependentdisplacement (207) of the variable capacitor (117) changes. Constraintscan consist of small pins or simply glue or epoxy along an edge, part ofan edge, or other point of constraint of the plate (206.)

Note that the moving plate (210) can comprise a cantilever as shown inFIG. 2B or any bar structure with various boundary conditions (both endsfixed, both ends free, etc.) as described in FIG. 2. Given thisdisclosure, it would be apparent to one skilled in the art, that otherboundary conditions on the moving plate can result in differentresonating modes. Some non-limiting alternative examples of movableplates in FIGS. 2B-2D (or movable bimorph plate(s) below) include: (1) abar constrained at one or more sides, (2) a plate of various shapesconstrained at zero or more points at the plate edge, and (3) a plate ofvarious shapes constrained at zero (unconstrained) or more points at theplate interior, and (4) any combination of the above. The constraintscan also include: constraints on the displacement and the derivatives ofthe displacement with respect to space (e.g. a sliding plate constrainedto one plane of motion) and time (velocity constrained at a point of theplate by clamping).

Another preferred embodiment utilizes magnetic fields as a modulatingsignal (103A) for exciting either the cantilever or the bar (FIGS.2B-2D) when this vibrating element, i.e. plate (202, 204, 206), containsa magnetic material. The modulating magnetic field is made to interactwith the cantilever or bar by way of a force which comes about bymagnetic-magnetic interaction. See FIG. 2B. This type of interaction canoccur when a DC field gradient is present in the region of the acmodulating field. Alternatively, as shown in FIG. 2B the fixed capacitorplate (201) on the substrate can be fabricated from a hard magneticmaterial put into a remanent state that is a magnetized state of thematerial having a field B_(R). In one embodiment, the AC interrogatingfield will cause a soft magnetic cantilever to vibrate and thus tomodulate the carrier. Maximum modulation is again achieved when the acfield is applied at the same frequency as the mechanical resonance ofthe cantilever.

In one such embodiment, a configuration typically found in reed switchesis used, the configuration consisting of two closely separated bars orribbons of magnetic material, preferably soft magnetic material for thepresent application, shown in FIG. 2E. The two elements of the switchcomprise at least one cantilever (202) which overlaps in part the secondmember (212). The magnetic force comes about through the polarization ofthe two members in the same direction so that in the overlapping region(213) the members have opposite magnetic polarity. An externally appliedsinusoidal ac field causes the polarity of the two elements to reversebut the reversal is such as to always cause the two elements to attract.Thus, in the absence of an external bias field, the ac modulationfrequency will be twice that of the applied sinusoidal field. In thepresence of a bias field greater than the absolute value of themodulating field, the modulated frequency will equal that of the appliedmodulating field. However, the operation of the reed switch is notlimited to any particular applied waveform. These switches can operateboth with and without an external bias field. In either ease, thevibration of at least one member of the switch produces a capacitivechange. Maximum modulation will occur when the attractive force betweenthe two reed switch members (202, 212) is at the same frequency as themechanical resonance of one of the reed switch elements.

See FIG. 3. In another preferred embodiment, a magnetostrictive materialconsisting of a thin foil (301) of soft magnetic material (for example,Metglas 2605SC with a composition of FeBSiC and 2605S2 with acomposition of FeBSi, Allied Signal) is attached to a second material(302) to form a vibrating bimorph (310). This bimorph resonates atmechanical resonance frequencies determined by the mechanical propertiesdescribed above. Because magnctostriction is an even function of theapplied magnetic field, that is the elongation or constriction of themagnetostrictive material is independent of the polarity of the appliedfield, the resonating bimorph (310), in the absence of a DC bias field,produces a modulation in the carrier at twice the modulating signal(103A) frequency due to the time dependent capacitance changes in thespacing of the capacitor caused by the mechanical variation (307). Inthe presence of a DC bias field with magnitude greater than the absolutevalue of the applied modulating field, the modulation of the carrierwill be equal in frequency to that of the applied field.

It should be clear to those skilled in the art that there also existhard magnetostrictive materials which possess very largemagnetostrictive coefficients such as terfenols which consist typicallyof a rare earth element with iron. However such materials require highmagnetic fields for their maximum magnetostriction to occur and thus arenot very practical for tag applications in which people are part of theenvironment in which the tags are used. However, such materials can beused as well in the manner described herein.

It is also possible to make variable capacitors with a magnetostrictiveelement without the use of a bimorph configuration. In that case, a DCbias field is required that is non-planar.

For optimal operation, the bimorph utilizes an additional DC field inorder to be excited by a modulating field/signal (103A). The DC fieldcan be provided by an externally applied bias field B_(b) (306), or bythe use of a magnetic material of high coercivity placed in the vicinityof the vibrating element. A preferred embodiment of the variablecapacitance (117) comprises a bimorph cantilever plate (310) fixed atone end and separated from a fixed capacitor plate (303) by anelectrically insulating cantilever support (304). The bimorph cantilever(310) is fabricated from a soft magnetostrictive foil (first layer 301)such as Metglas 2605CO (a trademark of the Allied Signal Corporation)attached to a hard magnetic material (second layer 302) such as iron,cobalt, nickel and any other ferromagnetic material (302). In thispreferred embodiment, the magnetized hard magnetic material (302) servesto provide the DC bias field. However, other non-magnetic materials canalso be used to provide a differential bending force (shear force) forthe bimorph cantilever (310) when the soft magnetostrictive foil changesin length. In these embodiments, the magnetic field is providedexternally (306). The external bias field (306) can be provided by abias on the modulating signal (103A) created by the modulating signalgenerator (102) and modulating signal antenna (103) and/or by anexternal magnetic field source.

Any of the movable plates described in FIG. 2 can be bimorphs.

Bimorph transponders/tags can be deactivated by demagnetizing the ironelement in which case the tags are reusable. They can also bedeactivated by disabling the capacitor or a portion of the coil circuita strong enough signal to open circuit these elements, in which case thetransponder/tag is not re-usable. Transponders can be made with multiplebimorphs (see multibit tags below) each with a second layer of adifferent hard magnetic material (302). In these bimorph configurations,different bimorph elements can be independently activated or deactivatedusing magnetic fields of different intensities applied to the individualhard magnetic elements, thereby providing a unique code on thetransponder/tag. For example, to activate an individual bimorph, amagnetic field is locally applied to the bimorph (e.g. using a magnetichead) to put the hard magnetic element into a remanent state. Todeactivate all bits (bimorphs) of the transponder/tag, the entire tag isplaced in a region of a time decrementing oscillatory magnetic fieldthat forces the hard magnetic element of each bimorph to a zero or nearzero remanent state. Alternatively, the time decrementing oscillatorymagnetic field can be locally applied to one or more of the bimorphs onthe transponder/tag to deactivate individual bimorphs (bits.) This canbe undertaken most conveniently at the tag manufacturing site.

Multibit transponders/tags can be made by using a single RLC circuit(125) which includes more than one variable capacitor component (117).In FIG. 4, an example of a multibit tag preferred embodiment (400) isshown with three bits embodied by three elements (401, 402, 403) each ofwhich is a variable capacitor (117). These variable capacitors (117) canbe an array of independent bars or any combination of variable capacitor(117) described in this disclosure that act as parallel capacitors inthe RLC circuit.

When set to resonate by way of acoustic excitation, e.g., by themodulating signal (103A), the resonating bimorph (310) produces amodulation in the carrier frequency at the modulating signal (103A)frequency (300) due to the time dependent capacitance changes caused bythe mechanical variation (307). Details of the bimorph have beendescribed previously in a U.S. patent application Ser. No. 08/344,771entitled “Multibit Bimorph Magnetic Tags Using Acoustic or MagneticInterrogation” by A. Schrott et al, filed Nov. 23, 1994 which is hereinincorporated by reference in its entirety.

Since the difference in amplitude between resonance and off-resonancevibration is substantial, it is possible to design a multibit tag inwhich each capacitor 117 (bar) can be modulated independently as eachhas a distinct set of mechanical resonant frequencies determined byboundary conditions and other parameters of the bar as described above.In this manner, each independent bar can modulate the resonant circuit(125) at a distinct modulating signal frequency (103A) by varying eachof the variable elements (401-403) the most when the modulating signalfrequency (103A) equals one of the mechanical resonance frequencies of agiven capacitor (401-403). As explained in U.S. patent application Ser.No. 08/344,771 to Schrott et al. each bar can be excited sequentially orsimultaneously by a linear combination of one or more excitationfrequencies with independent amplitudes. If a linear combination is usedas the modulating frequency (103A), a frequency analyzer, e.g.,(incorporated with block 107) is required to separate the frequenciesafter demodulation.

The variable capacitors can each be designed to have the same value ofcapacitance in their unexcited state (quiescent capacitance) by scalingthe bars' dimensions and their respective separation, or gap 200G,between bar (202, 204, 206) and the lower plate capacitor element.Typically, modulation of each individual bar at its mechanical resonantfrequency results in a total capacitance change of approximately 2%. Forexample, a bar as small as 5 mm×0.5 mm×0.05 mm and a gap (200G) of 0.025mm has a mechanical resonance at a modulating signal (103A) frequencythat can be readily detected. Such changes cause an RF carrier signal(104A) modulation that is easily detected with standard amplitudemodulation receivers (110). The gap (200G) inherently has a gap sizewith is the distance between the two plates (e.g. 201 and 202.) The gapsize is changed by the vibration of one or more of the plates.

With these small sizes, an array of 20 bars (cantilevers) can be easilyaccommodated in area on the order of 1 cm squared to produce a (20 bit)multibit transponder (400). Since each bar (cantilever) (401, 402, 403,. . . etc.) responds independently to distinct mechanical resonance(excitation) frequencies, a code can be established consisting of “1's”and “0's”. A ‘1’ can be assigned if excitation, i.e. the modulatingsignal (103A), at a given modulation frequency results in modulation ofthe RF carrier signal (104A). This is determined if the given modulationsignal (103A) is detected at the receiver (110) indicating a “1”.Similarly, failure to produce such modulation at a given frequencycorresponds to a “0”. As explained above, when the bars comprise amagnetic material, they can be excited by a set of magnetic modulatingfrequencies. Alternatively, a set of acoustic waves of appropriatefrequencies can be used as the modulating signal (103A) to causemechanical resonance of the variable capacitors.

Other preferred embodiments for modulating (varying) the capacitancecomprises two plates whose overlapping area can be varied (See FIGS. 5Aand 5B). In these embodiments, the spacing (or gap size 200G) betweenthe plates need not vary (207, 307). Capacitance varies by allowing atleast one of the plates (501, 502) to slide or change its overlappeddimension (505) typically by sliding (510) one or both of the plates(501, 502) with respect to one another. In some preferred embodiments,this sliding (510) can be accomplished by the use of magnetostrictionwhere the variable capacitor comprises two strips (501, 502) ofmagnetostrictive material, preferably though not necessarily biased witha small piece of magnetized hard magnetic material as described above.Examples of this magnetostrictive material include iron rich alloys suchas Metglas 2605CO or other amorphous alloys of iron or cobalt.

See FIG. 5A. In a preferred embodiment, the magnetostrictive coefficientof each plate (501, 502) is chosen to maximize the change in overlap(505) upon excitation with the modulating signal (103A). Each slidingelement (501, 502) is pinned (501A, 502A, respectively) at one end andthe two elements are separated from one another by a thin dielectric(503), such as polyimide. The presence of modulating signal (103A)produces a mechanical oscillatory motion (variation) (515) of the platesdue to the magnetostriction. The mechanical oscillation varies thecapacitance (117) which modulates the resonance of the RLC circuit. Notethat in this embodiment, there are no mechanical resonant frequencies ofthe mechanical oscillation (the mechanical oscillation of the variablecapacitor is forced by the modulating signal field 103A, as anon-resonance response, because there is no restoring force on theplates (501, 502).

In FIG. 5B, an alternative preferred variable sliding capacitor (117) isin this case formed by using a single magnetostrictive element (528)sliding (530) over but separated by a dielectric (503) from a fixedmetallic plate (532) deposited or attached to the substrate (534). Aportion (538) of the element (528) may be magnetostrictive. For example,by making only the elongated portion (538) magnetostrictive, the entireelement (538) will slide (530).

Again, the change in capacitance with time is caused by the change inthe quiescent amount of area of overlap (535A) which goes into theinstantaneous overlap (535B) of the upper (528) and lower (532) plates.As before, when no external restoring force is applied, the structure(528) mechanically oscillates but does not mechanically resonate, i.e.the response is non-resonance. Because of sliding friction, theamplitude of the sliding (530), i.e. the non-resonance response, isdamped at high frequencies, typically 2 kHz and above. Therefore themodulating frequency range of the modulating signal (103A) includesfrequencies below 2 kHz. Operation at higher frequencies is possible butwith a lower sliding amplitude (530) since as the modulating signal(103A) frequency increases and the lower sliding amplitude produces asmaller variation in capacitance. The friction can be reduced, and hencethe frequency of operation increased, by using a dielectric (503) with alow coefficient of friction such as teflon or polyimide. (Teflon is atrademark of the Dupont Corporation.) Alternatively, one can use alubricant in the region where sliding occurs.

If a restoring force is supplied by way of an elastic constraint (545),FIG. 5C, a mechanical resonance can be established where the mechanicalresonant frequency is directly related to the elastic constant andinversely related to the mass of the constraining material. This elasticcoupling link (545) can consist of an elastic adhesive for attachment ofthe magnetostrictive material (541) such as a thin film or foil (541) tothe substrate (546). Elastic adhesive materials used for the couplinglink (545) include: silicone, elastomers, or synthetic rubber adhesives.As before, the capacitive change leading to modulation can be achievedby a change in the overlap (547) of the movable thin foil (541) withanother fixed element (542), the latter (542) forming the second plateof a capacitor. Alternatively, the capacitance change can arise from aslight bowing or bending of the magnetostrictive element (541) withrespect to the lower plate (542), thereby changing the spacing (gap/gapsize 200G) between the capacitor plates (541, 542). In either case, theelastic constraint (545) gives rise to a mechanical resonance and theensuing change in capacitance. Again, the plates are separated by adielectric (503).

An array of such capacitive components can be fabricated to form amultibit tag (400) where each component can be made to resonate at adistinct mechanical frequency by providing, for example, elasticcoupling (545) or links of varying elastic constants. (Note that thearray can include any of the other component embodiments disclosedherein.) Each magnetostrictive element (541) can have a different mass.Alternative preferred embodiments include the magnetostrictive element(or elements 541) being pinned at one end (548) and elasticallyconstrained (545) at the other by a leaf or coil spring. The mechanicalresonance results from the restoring force on the magnetostrictiveelement (541). The frequency of the mechanical resonance is a functionof the restoring force and the mass of the moving element and thereforecan be changed for each component (bit) by changing parameters such asspring constant and the mass of the moving element.

The ensuing circuit modulation resulting from the capacitance change ofeach of one or more variable capacitors can be utilized to establish acode where the presence of each mechanical resonant frequency thatcorresponds to a “1”, and the absence of a mechanical resonance at aparticular frequency corresponding to a “0”. The presence of each of themechanical resonant frequencies, and hence, the coded binary informationcan be detected by exciting the transponder with various frequencies ofthe modulating signal (103A) and using the receiver (110) to determinewhether or not the RF carrier signal (104A) was modulated at a givenmodulating signal frequency (103A). In an alternative preferredembodiment, the modulating signal generator (102) will cause themodulating signal (103A) to sweep through a range of frequenciesincluding all (or a set of) the mechanical resonant frequencies of themultibit transponder (400). Again, presence (absence) of a modulatedfrequency at the receiver (110) indicates a “1” (0″) in a binary code.

Modulation can also be achieved by the use of a non-resonant gap change.(See FIG. 5D). Here, the gap change (555), i.e., the change in the gapsize (200G), is caused by a sliding magnetostrictive element (551) thatforms a non-resonant variable gap (555) between the first plate (551)and a second plate (552) that is either fixed or also capable ofsliding. Upon application of the modulating signal (103A), thenon-resonant variable gap (555) changes at the frequency of themodulating signal (103A). This constitutes a variable capacitor (117)that changes the electrical resonance of the RLC circuit (125) causingthe carrier signal (104A) to be modulated at the frequency of modulatingsignal (103A).

FIG. 5E shows a preferred embodiment in which two movablemagnetostrictive plates (561A,B) are positioned orthogonally (angle 565)over a fixed plate (564). The fixed plate (564) is separated from eachof the movable plates (561A,B) by a dielectric (503). In thisconfiguration the transponder/tag (150) is less susceptible to thedirection of the modulating signal (103A) since there are now twoorthogonal positions that can be aligned with the modulating signal(103A). This embodiment makes the overlap (505A, 505B) variation lessdependent on the orientation of the device (560). The angle (565)between the first (561A) and second (561 B) plate can be other than 90degrees, i.e., non orthogonal.

FIG. 6A shows a transponder/tag (150, 600) where the modulating elementof the RLC circuit (125) is a variable inductor. The variation isachieved, for example by using a ferromagnetic sheet or shim (601),preferably of low coercivity, placed in proximity to a section of a coil(inductor) (602) and in a preferred device (600), next to a fixed, i.e.non-variable, plate capacitor (603, 114). The inductor coil (602) andferromagnetic element (601) are electrically separated. In thisconfiguration, an applied modulating signal (103A) can drive theferromagnetic element (601) in and out of its non linear permeabilityrange as it sweeps out a portion of its hysteresis curve causing achange in the inductance of the coil (602). Since the inductance of acoil is equal to the flux linking an element of coil divided by thecurrent through the coil (here the current is provided by the carrierfrequency field 104A) the carrier signal (104A) will become modulated.Since the ferromagnet (601) is not a resonant element, any modulatingfrequency, typically in the audio frequency range, can be used tomodulate the carrier.

Alternatively, FIG. 6B shows a bank (631) of one or more cantilevers(201) constructed from a magnetic material which can couple flux througha coil on the substrate of the RLC circuit. As already described, themechanical resonance of the cantilevers (201) can be utilized tomodulate the carrier by changing the capacitance. In a similar manner,when the cantilevers are disposed near a coil, as one or moreferromagnetic sheet elements each cantilever (201) can be made to varythe inductance of the circuit by changing the flux coupling eitherthrough the displacement associated with its vibration or through thechange in permeability of the cantilever due to magnetoelastic effects.In this manner, a multibit modulated tag (630) can be establishedthrough inductive changes with each modulation frequency controlled by amechanical resonance of a cantilever (201).

In another embodiment (650) shown in FIG. 6C, more than one coil is usedto produce variable mutual inductance (670) modulation by changing thepermeability of a shim (601) used to couple the flux shared by two ormore mutually coupled coils (660, 665). In an alternative embodiment, inFIG. 6D the shim (601) is replaced by a bank (631) of cantilevers (201)that vary the coupling flux (670) by changing the proximity of themechanically vibrating plate (201) to the coils, thereby modulating theamount flux coupled to the coils.

As shown in FIG. 7A, it is also possible to modulate the, impedance ofthe transponder/tag (150) by inserting into the circuit a small sectionof material possessing magneto-impedance (706), for example, a FeCoSiBwire. The impedance is modulated by use of a low frequency magneticfield, the modulating field/signal (103A), which results in a modulatedcarrier signal (104A). In this preferred embodiment, the impedance beingvaried is complex, i.e., both resistive (115) and inductive (116)components vary.

In another embodiment, FIG. 7B, the impedance variation is primarilyresistive (115). Here a change in resistance with magnetic field,modulating field/signal (103A), causes modulation in the carrier (104A)by changing the Q (proportional to the dissipation or loss at resonance)of the resonant circuit (125) resulting in the amplitude change of thesignal at the modulating frequency. For this to occur, a metal orsemiconductor exhibiting magnetoresistance is inserted in the circuit(125) as the variable resistance (115), typically in series with thecoil (113). Metals or semiconductors exhibiting these effects includecertain oxides of Mn known as colossal magnetoresistive materials(CMR's). Other magnetoresistive materials include: Bi, Cd, and Sb.

The RLC circuit can be made by simple metal deposition on an insulatingsubstrate followed by etching techniques, well known by those skilled inthe art. The cantilevers and sliding elements can be fabricated bymicromachining or stamping techniques. Magnetic films can be disposedonto cantilevers to form bimorphs by lamination, plating, vapor orplasma deposition. The moving assembly can be attached to the substrateby means of any number of known adhesives. Since all motion or movementsof elements of the assembly involve small displacements, the modulatingassembly can be encapsulated without interfering with the motion of theassembly or its elements.

FIG. 8 is a schematic of the interrogator (801) and the receiving units(810). The interrogator (801) contains an RF generator (802) which sendsout a periodic or continuous RF signal (804A), typically in themegahertz frequency range. In addition, a second generator (803)transmits a lower frequency modulating signal or series of modulatingsignals (103A) at one or more frequencies, preferably in the audiorange, to modulate the variable circuit element of the transponder/tag(150). The interrogating signals are propagated by suitable antennas(804, 805) such as single loops of wire to excite the tag resonantcircuit (125) with both a carrier (804A) and a modulating signal (103A),In the case of acoustic excitation, (805) corresponds to a loudspeaker.The receiving coil (806) detects only the carrier frequency (804A) inthe absence of a tag within its operative range. In that case, thereceiver (807) rectifies (demodulates) the carrier which gives rise to adc signal and no further response by the detection circuit, i.e., thedetector receives no coded signal. In the presence of a tag, eithersingle or multibit, the carrier is amplitude modulated (855) with one ormore modulation frequencies of the modulating signal (103A). Formultibit tags, a computer (808) determines which modulation frequenciesare present in the modulated carrier (855) by detecting the signal usingfor example, fixed band pass filters (809). If interrogation usesmultiplexed signals, the receiver (801) can determine the tag code bycomparing transmitted voltages at pre-determined frequencies thatestablish a code.

In an alternative embodiment of system (800), frequency modulation (FM)can also be used for interrogation and demodulation. For example, FM canbe achieved by the use of a feedback circuit (850) linking the carriertransmitter and the tag signal receiver. This feedback (850) is made tochange the frequency of the carrier (804A) to maintain a constantamplitude of the modulated signal (855) at the receiver (810). Thevariation of frequency of the carrier signal (804A) constitutesfrequency modulation (FM) at the frequency of the modulating signal(103A). The frequency variation can be achieved by a voltage controlledoscillator (VCO) having a voltage control signal determined by thefeedback signal (850), e.g. the difference between the amplitude of themodulated signal (855) and a set point.

FIG. 9 is a flow chart showing the method steps performed further by thetag (150), interrogation and sensing system. The interrogationconsisting of sending (910) the RF carrier signal (104A) and sending(920) the modulating signal (103A) simultaneously from separate antennas(104,103). The tag (150) is interrogated via these signals i.e., thetransponder (150) modulates (930) the carrier (104A) with the modulatingsignal (103A). A pickup antenna (105) detects (940) the modulated signal(155). The modulated carrier (155) is demodulated (950) by a receivertypically an AM receiver, with the signals sensed by a frequencyanalyzer (960) or a predetermined set of narrow bandpass filters (960).These signals are compared with those frequencies in memory (906) of acomputer. The frequencies in the computer memory are those frequenciesthat correspond to the set of resonant frequencies assigned to allpossible bits of the tag. For a multibit tag, the computer comparisonenables the item to be identified since the presence or absence offrequencies, ƒ_(i) can correspond to a ‘1’, ‘0’ code respectively.

After interrogating the entire set of frequencies within the computermemory (907), information about an item attached to the transponder/tag(150), gives the identity and/or status (908) of the item. For example,in an electronic article surveillance (EAS) application, the statusindicates that the item has not been paid for and an alarm (909) will beactivated. The computer (107, 808) will also record what item is beingstolen. This type of data can be useful in providing informationregarding frequency of attempted theft leading to necessary theftprevention countermeasures.

In one specific use in a retail environment, the tag (150) will bedeactivated at a point of sale when tendering for the item to which thetag is attached occurs. If the item is not paid for, the tag will not bedeactivated, i.e., the tag remains active. Active tags are sensed at anexit of the store using method (900) which causes an alarm to sound.Once the item has passed through the interrogation gate and beenidentified or once the theft alarm has sounded, the system is reset andthe modulation frequency registers are cleared for passage of the nexttag.

The circuits can be disposed on a flexible substrate suited forattachment to small items, such as those used today in retail pharmaciesand groceries. They can be encased in a paper or cardboard cover. Inthese applications, the tags (150) need not be configured for re-use andcan be deactivated destructively. For tags on more expensive items, thetag and its substrate can be encased in a plastic covering as is wellknown in the art, allowing it to be removed from the attached articleand reused. Here deactivation can be non-destructive if desired or thetag may be left in the active state for reuse.

In an alternative preferred embodiment, the modulating signal (103A) isnot transmitted. In this embodiment, the transponder/tag (150) willrespond by absorbing (104AB) the highest amount of energy from thecarrier signal (104A, 108A) when the carrier signal frequency is equalto the resonant frequency of the resonant circuit (125). By sensing this“dip” in the RF signal (104A, 804A), the system 800 (175) can determinethat a transponder/tag (150) is within the range of the RF signal (104A,804A).

Personalization of the modulated RF tag containing cantilevers orvibrating bars can be achieved in a number of ways using computercontrol. Since modulation occurs due to mechanical resonances of amodulating elements, e.g., cantilevers or bars, personalization isachieved by selectively enabling or disabling any resonating memberproducing modulation. In general this can be achieved either bymechanical means or by altering the magnetic coupling of the resonatingstructure with respect to the modulating magnetic field.

FIG. 10, is an illustration of the method for personalizing thecantilevers or bar of tag (150) utilizing magnetic means discussed inthe description of FIG. 3 above. Here, each cantilever (1010, 201) hasan individual bias magnet (1001) that can be magnetized or demagnetizedusing a magnetic head (1002). In a preferred embodiment the magnetichead contains multiple elements (1003) that fit over each bias elementof the vibrating members of the tag. The computer is programmed toactivate the head in such a way that it magnetizes or demagnetizes themagnetic cantilever element. Activation is achieved by means of a localdc field while deactivation is accomplished by a local decrementing acmagnetic field. To avoid cross talk, a preferred method of using thismulti-element head is to address each element of the head sequentiallyrather than simultaneously. As known in the art of recording magneticmedia, the close proximity of the head (1002) to the bias magnet (1001)only effects that respective bias magnet (1001) to which the head (1002)is near.

Mechanical means includes a locking/unlocking mechanism for eachindividual vibrating member. This is shown in an embodiment in FIG. 11.The tag (150) includes a thin cover (1101) which contains dimpled areas(1102), positioned over each vibrating element (1010) in a region inwhich vibration occurs. The dimple (1102) can be positioned to be incontact with the vibrating element (1010) in which case vibration isfrozen, i.e. the element is disabled. Conversely, the dimple can be in aposition such that no contact is made with the vibrating element therebyleaving the element enabled.

Personalization is achieved by selecting the elements to be enabled ordisabled. The cover (1101) is fabricated from a material that canlocally be deformed without damage by applying a sufficient pressure,either by mechanical means or air pressure or combination of mechanicaland air pressure. This material preferrably should be capable of beinginjected molded, e.g., polyvinyldifluoride (PVDF) orpolytetrafluoroethylene. The personalizing head (1103) contains one ormore elements (1109) that can access each of the vibrating elements tobe personalized by either pushing, pulling or providing positive ornegative air pressure to change the dimple curvature.

Alternatively, the dimple can be pivoted so that pushing in either oneof two locations will alter its state or position relative to thevibrating element, thereby enabling or disabling the vibrating element.

Alternative equivalent embodiments, within the scope of this inventionand within the contemplation of the inventors will become apparent toone with skill in the art that is given this disclosure.

We claim:
 1. A transponder comprising a resonant circuit having two ormore electromagnetic energy storage components that determine a circuitresonance frequency of the resonant circuit, at least one of theelectromagnetic storage components varying at a component frequency inresponse to an external modulating energy field so that the circuitresonance frequency changes.
 2. A transponder comprising: a resonantcircuit having two or more electromagnetic energy storage componentsthat determine a circuit resonance frequency of the resonant circuit, atleast one of the electromagnetic storage components varying continuallyin response to an external modulating energy field so that the circuitresonance frequency changes in response to the external modulatingenergy field; and an electromagnetic carrier field with a carrierfrequency and having an absorbed part, the absorbed part being an amountof energy of the carrier field being absorbed by the resonant circuit,the absorbed part being greatest at the circuit resonance frequency. 3.A transponder, as in claim 2, where the carrier field further comprisesa reflected part and a transmitted part, the reflected part being areflected amount of energy of the carrier field being reflected from theresonant circuit and the transmitted part being a transmitted amount ofenergy of the carrier field that passes through the resonant circuit,where all of the energy of carrier field equals the sum of the absorbpart, the reflected part, and the transmitted part and where changes ofthe circuit resonance frequency modulate the absorbed part, reflectedpart, and transmitted part.
 4. A transponder, as in claim 1, where theresonant circuit has one or more variable capacitors and an inductancethat determine a circuit resonance frequency of the resonant circuit, atleast one of the variable capacitors varying continually in response toan external modulating energy field so that the circuit resonancefrequency varies continually.
 5. A transponder, as in claim 4, where atleast one or the capacitors is a plate capacitor having one or morefirst plates and a second plate, the first and second plates beingseparated by a gap having a gap size, and where at least the first platehas a movement in response to the external modulating energy field, themovement causing the capacitance of the capacitor to vary.
 6. Atransponder, as in claim 5, where one or more of the first plates is aconstrained plate being constrained at zero or more points by aconstraint and the constraints determine one or more mechanicalresonance frequencies of the constrained plate.
 7. A transponder, as inclaim 6, where the movement is a non-resonance response to the externalmodulating energy field and the external modulating energy field has afrequency not equal to any of the mechanical resonance frequencies ofthe constrained plate.
 8. A transponder, as in claim 5, where themovement of the first plate causes a change in the gap size between thefirst and second plate.
 9. A transponder, as in claim 8, where theexternal modulating energy field has a modulating frequency and themovement is a vibration of the first plate at the modulating frequency.10. A transponder, as in claim 9, where the change in the gap size isgreatest when the modulating frequency is equal to any one of themechanical resonance frequencies.
 11. A transponder, as in claim 8,where the external modulation energy field is an acoustic field.
 12. Atransponder, as in claim 6, where the constrained plate is a cantileverhaving a first and second end and the first end is constrained by beingfixably attached to a support and the second end is unconstrained topermit the movement, the movement being a vibration changing the gapsize.
 13. A transponder, as in claim 6, where the constrained plate is abar having a first and second end and the first end and second end areconstrained by being fixably attached to a support, the bar furthercapable of a transverse mechanical motion at one or more of themechanical resonance frequencies in a direction that changes the gapsize.
 14. A transponder, as in claim 6, where the constrained plate isconstrained at zero points, therefore being an unconstrained platecapable of a transverse mechanical motion at one or more of themechanical resonance frequencies in a direction that changes the size.15. A transponder, as in claim 5, where the first plate is a bimorph,the bimorph having a first and second layer.
 16. A transponder, as inclaim 15, where the external modulating energy field includes any one ofthe following: an acoustic field, an electromagnetic field varying at amodulating frequency, and an electromagnetic field varying it amodulating frequency with a DC magnetic bias.
 17. A transponder, as inclaim 15, where the first layer is made of a magnetostrictive material.18. A transponder, as in claim 17, where the second layer is made of acoercive material left in a remanent state with a coercivity greaterthan an amplitude of the external modulating energy field.
 19. Atransponder, as in claim 18, where a change in gap size is greatest whena modulating frequency of the external modulating energy field is equalto a mechanical resonance frequency of the first plate.
 20. Atransponder, as in claim 17, where the external modulating energy fieldis an electromagnetic field and the change in the gap size is greatestwhere a modulating frequency of the modulating energy field is equal toone half of the frequency of one the mechanical resonance frequencies ofthe first plate.
 21. A transponder, as in claim 6, where the externalmodulating energy field is a time varying magnetic field and the each ofthe first plates and each of the second plates have an area of overlap,the first and second plates consisting of soft magnetic material, thefirst plates being cantilevers with a first and a second end, the firstend being constrained and the second end being free to move, the secondend of the first plate being attracted to the second end of the secondplate with a time varying force due to the presence of the time varyingmagnetic field, the time varying force causing a mechanical vibration ofthe plates, the capacitance of the capacitor formed by these two plateschanging due to a change in the gap size caused by the mechanicalvibration.
 22. A transponder, as in claim 6, where the energy field is atime varying magnetic field and the first plate is in the presence of anon-uniform magnetic bias field, the first plate comprising any one ofthe following: a magnetostrictive material and a soft magnetic material.23. A transponder, as in claim 22, where the magnetic bias field issupplied by any one of the following: an externally applied spatiallynon-uniform magnetic field and the second plate being made from a highcoercivity material left in a remanent state.
 24. A transponder, as inclaim 5, where at least a portion of the first plate is capable ofslidably moving with respect to the second plate to change the gap size.25. A transponder, as in claim 5, where, in response to the externalmodulating energy field, at least a portion of the first plate and aportion of the second plate are capable of slidably moving with respectto one another to change the gap size.
 26. A transponder, as in claim24, where the first plate is made of a magnetostrictive material and iscaused to slidably move when excited by the external modulating energyfield, the external modulating energy field being a magnetic fieldvarying at a modulating frequency.
 27. A transponder, as in claim 26,where the first plate is attached to an elastic link that provides arestoring force allowing the first plate to mechanically resonate at oneor more mechanical resonance frequencies in response to the modulatingmagnetic field.
 28. A transponder, as in claim 5, where the first plateand the second plate are configured to have an area of overlap, and themovement of the first plate causes a change in the overlap area inmanner to vary the capacitance of the capacitor.
 29. A transponder, asin claim 28, where the first plate is made of a magnetostrictivematerial and slides in response to the external modulating energy field,the external modulating energy field being an electromagnetic field. 30.A transponder, as in 28, where the first plate is constrained by anelastic link that provides a restoring force giving rise to one or moremechanical resonant frequencies of the first plate.
 31. A transponder,as in claim 30, where the overlap change is the greatest under any oneof the following conditions: a. a modulating frequency of the externalmodulating energy field is equal to one half of one of the mechanicalresonant frequencies, and b. the external modulating energy field has anAC part and a DC part, the DC part is greater than a peak amplitude ofthe AC part and the modulating frequency is equal to one of themechanical resonant frequencies.
 32. A transponder, as in claim 28,where two or more of the first plates slide in a plane parallel to thesecond plate, each of the first plate sliding directions beingnon-parallel with one another.
 33. A transponder, as in claim 1, whereone or more of the electromagnetic storage components is a variableinductor comprising: (a) a coil; and (b) a flux changing element, theflux changing element varying the inductance of the variable inductor bychanging a flux of the variable inductor non-linearly with respect to achange of current flowing in the variable inductor.
 34. A transponder,as in claim 33, where the flux changing element is a ferromagnetic sheetthat is driven in and out of a non-linear range of a hysteresis curve ofthe ferromagnetic sheet by the external modulating energy field.
 35. Atransponder, as in claim 33, where the flux changing element is one ormore moving plates in proximity to the coil, where said plates areconstrained at zero or more points by a constraint and the constraintsdetermine one or more mechanical resonant frequencies of the constrainedplates, the plates being moved by the external modulating energy field.36. A transponder, as in claim 33, where the flux changing element isone or more mutual inductances coupled to the coil by the flux.
 37. Atransponder, as in claim 36, where the coupling is varied by aferromagnetic sheet that is driven in and out of a non-linear range of ahysteresis curve of the ferromagnetic sheet by the external modulatingenergy field.
 38. A transponder, as in claim 36, where the coupling isvaried by one or more moving plates in proximity to the coil, where saidmoving plates are constrained at zero or more points and the constraintsdetermine one or more mechanical resonant frequencies of the constrainedplates, the plates being moved by the external modulating energy field.39. A transponder, as in claim 1, where one or more of theelectromagnetic storage components is a variable impedance, the variableimpedance being a material exhibiting magneto-impedance changed by theexternal modulating energy field, the external modulating energy fieldbeing an electromagnetic field.
 40. A transponder, as in claim 39, wherethe variable impedance is made of a magnetoresistive material includingany of the following: permalloy, Sb, Bi, non-magnetic/magnetic layeredcompounds such as copper-permalloy, manganese oxides.
 41. A transponder,as in claim 39, where the material is a soft magnetic wire comprisingthe alloy FeCoSiB.
 42. A transponder comprising: a resonant circuithaving two or more electromagnetic energy storage components thatdetermine a circuit resonance frequency of the resonant circuit; anexternal modulating energy field, having one or more modulatingfrequencies, one or more of the electromagnetic energy storagecomponents being a varying component, the varying component being variedat a component frequency by the external modulating energy field, thevarying component causing the circuit resonance frequency to change; andan electromagnetic carrier field with a carrier frequency, the carrierfrequency being modulated with one or more component frequencies toencode information on the carrier frequency.
 43. A transponder, as inclaim 42, where the information is a binary code having one or morebits, each bit having a bit value, and each bit values capable ofindicating the presence and absence of one of modulating frequencies.44. A system for obtaining information from a transponder, comprising: abase station that transmits an electromagnetic carrier field with acarrier frequency and an external modulating energy field, the externalmodulating energy field having one or more modulating frequencies; atransponder having a resonant circuit with two or more electromagneticenergy storage components that determine a circuit resonance frequencyof the resonant circuit, one or more of the electromagnetic energystorage components being a varying component, each of the varyingcomponents being varied at a component frequency by one of themodulating frequencies, the varying component causing the circuitresonance frequency to change thereby modulating the carrier field withone or more component frequencies to encode information on the carrierfield; and a receiver for receiving the modulated carrier field anddetecting the information on the carrier field.
 45. A system, as inclaim 44, where the carrier field is modulated with amplitudemodulation.
 46. A system, as in claim 44, where the carrier field ismodulated with frequency modulation.
 47. A system, as in claim 44, wherethe transponder is a tag on an object and the information describes theobject.
 48. A system, as in claim 44, where the information is a binarycode having one or more bits, each bit having a bit value, and each bitvalues capable of indicating the presence and absence of one ofmodulating frequencies.
 49. A system, as in claim 44, where the externalmodulating energy field includes any one of the following: an acousticfield, an electromagnetic field, and an electromagnetic field with amagnetic DC bias.
 50. A transponder comprising a resonant circuit havingtwo or more electromagnetic energy storage component means fordetermining a circuit resonance frequency of the resonant circuit, atleast one of the electromagnetic storage component means varying at acomponent frequency in response to an external modulating energy fieldso that the circuit resonance frequency changes.
 51. A method ofcreating a modulated carrier field comprising the steps of: transmittingan electromagnetic carrier field and an external modulating energyfield, the electromagnetic carrier field having a carrier frequency andthe external modulating energy field having one or more modulatingfrequencies; and modulating the carrier field with a transponder, thetransponder having a resonant circuit with two or more electromagneticenergy storage components that determine a circuit resonance frequencyof the resonant circuit, one or more of the electromagnetic energystorage components being a varying component, each of the varyingcomponents being varied at a component frequency by one of themodulating frequencies, the varying component causing the circuitresonance frequency to change thereby modulating the carrier field withone or more component frequencies.
 52. A method, as in claim 51, wherethe the electromagnetic carrier field and the external modulating energyfield are transmitted by a base station.
 53. A method, as in claim 51,where the component frequencies modulated on the carrier field areinformation.
 54. A method, as in claim 53, where the modulated carrierfield is received by a receiver that demodulates the modulated carrierfield to obtain the information.
 55. A method for communicatinginformation from a transponder, wherein said transponder has a resonancefrequency which is variable and wherein information from the transponderis transmitted by varying the resonant frequency of the transponder toencode information on a carrier field incident on the transponder, saidmethod comprising: (a) transmitting electromagnetic energy to thetransponder, said electromagnetic energy having an electromagneticcarrier field with a carrier frequency, and an external energy field,the external energy field having one or more modulating frequencies, and(b) varying the resonant frequency of said transponder in response toone or more of the modulating frequencies.
 56. A system as in claim 55,wherein the transponder resonant frequency is determined by anelectronic circuit including a plurality of variable electronic storagecomponents.
 57. A system as in claim 56, wherein at least one of saidvariable electronic storage components is an inductor.
 58. A system asin claim 56, wherein at least one of said variable electronic storagecomponents is a capacitor.
 59. A system as in claim 56, wherein at leastone of said variable electronic storage components is a mechanicallyvariable component.
 60. A system as in claim 55, wherein each of saidmodulation frequencies corresponds to a set of data.
 61. A system as inclaim 55, wherein said varying components can be selectively chosen foroperation on the transponder via a transponder personalization method.62. A transponder comprising an electronic circuit having two or moreenergy storage components, said circuit having a non excited resonantfrequency defined by the electronic characteristics of said energystorage components when said variable electronic storage components arein an non-excited state, and said circuit having one or more excitedresonant frequencies defined by the electronic characteristics of saidenergy storage components when one or more of said variable electronicstorage components are in an excited state, wherein one or more of saidenergy storage components is a variable energy storage component whichcan be excited by an interrogation radiation field at a particularfrequency.
 63. A transponder as in claim 62, wherein at least one ofsaid energy storage components is a capacitor.
 64. A transponder as inclaim 62, wherein at least one of said energy storage components is aninductor.
 65. A transponder as in claim 62, wherein at least one of saidvariable energy storage components is a capacitor.
 66. A transponder asin claim 62, wherein at least one of said variable energy storagecomponents is an inductor.
 67. A transponder as in claim 62, whereineach of said excited resonant frequencies corresponds to a set of data.68. A transponder as in claim 62, wherein each of said variableelectronic storage components can be selectively chosen for operation onthe transponder via a personalization method.
 69. A transpondercomprising a resonant circuit having one or more magnetic componentsthat affect a circuit resonance frequency of the resonant circuit, saidtransponder being identifiable according to the characteristics of saidone or more magnetic components.
 70. A transponder comprising a resonantcircuit having one or more components that affect the circuit resonancefrequency of the resonant circuit when said transponder is interrogatedby an external field, said transponder being identifiable according tothe characteristics of said one or more components that affect thecircuit resonance frequency of the resonant circuit when saidtransponder is interrogated by an external field, and the transponderhaving a system for storing status as authorized or unauthorizedrespectively.